1. Field of the Invention
This invention relates to electrode materials in surface acoustic wave (SAW) devices which are required to have power durability, such as, for example, SAW filters employed in mobile phones and branching filters operable in a high-frequency band from 800-MHz band to GHz band.
2. Prior Art
Surface acoustic wave (SAW) devices, typically SAW filters and SAW resonators are prevalently utilized instead of dielectric filters as RF band filters in mobile communication equipment such as mobile phones and cordless phones. The reason is that the SAW devices, especially SAW filters have a smaller size than the dielectric filters and when devices of the identical size are compared, the former have better electrical characteristics.
The SAW device includes at least a piezoelectric substrate, a comb-shaped electrode pattern in the form of a metal film formed on a surface of the piezoelectric substrate, and a package accommodating the piezoelectric substrate and the electrode pattern. As the piezoelectric substrate, lithium niobate, lithium tantalate and rock crystal are used. Especially for RF band filters, lithium niobate and lithium tantalate are often used on account of their high electromechanical coupling constant.
Electrodes in SAW devices are generally formed of Al—Cu alloys. Especially for devices required to have power durability, an attempt was made to increase the Cu concentration of Al—Cu alloy. However, Al—Cu alloy materials having high Cu concentrations are susceptible to corrosion, especially after dry etching with chlorine gas. This prohibited consistent manufacture.
Attempts were also made to use as the electrode material Al—Ti, Al—Ta and other aluminum base alloys which are resistant to corrosion and have good power durability (see, for example, JP-B 7-107967 and JP-A 9-298442). However, these alloy materials are not applicable to high efficiency SAW devices since they have a higher electrical resistance than the Al—Cu alloys.
It was also attempted to improve power durability using a film consisting of alternately deposited layers of different metals, such as a four-layer film of titanium layers and Al—Sc—Cu alloy layers or a three-layer film of Al—Cu alloy layer and copper layer (see, for example, WO 99/54995 and JP-A 7-122961). In the case of an alternately deposited layer film for use in SAW devices operable below 1 GHz band, the thickness of aluminum alloy can be increased to 0.1 μm or greater. However, the resistivity of a thin film generally increases as the thickness decreases. Then, when the alternately deposited layer film is used as the electrode in a SAW device operable in a high-frequency band at or above 1 GHz, the total layer thickness is approximately 0.2 μm, each of the alternately deposited layers has a thickness of less than 0.1 μm, which gives an increased electrical resistance. Therefore, these structures encounter a limit in establishing high efficiency SAW devices.
When SAW devices are utilized as branching filters, power durability or tolerance is requisite. Under substantial oscillation by SAW, electrode materials having poor power durability tend to undergo migration and generate voids, hillocks and whiskers. The generation of voids causes the electrical resistance of electrode material to increase, eventually degrading the insertion loss of SAW devices. The generation of hillocks and whiskers causes shorts between electrode digits. The generation of hillocks and whiskers is believed due to local concentration of material by displacement around voids. Then by controlling the generation of voids, the generation of hillocks and whiskers can be suppressed and the increase of electrical resistance be suppressed.
WO 97/11526 discloses that the generation of voids in Al—Cu alloy is suppressed by localizing Cu atoms at grain boundaries for thereby improving migration resistance, and power durability is improved. This suggests that the migration resistance at grain boundaries must be enhanced before power durability can be improved.
Therefore, there is a demand for an electrode formed of a single crystal material which is essentially free of such weak grain boundaries.